How supermassive black holes were formed

The first supermassive black holes were formed shortly after the "Big Bang". That is the conclusion reached by an international research group led by Prof. Lucio Mayer from the University of Zurich. As the researchers write in "Nature", the supermassive black holes were formed through the collision of galaxies 13 billion years ago. The new findings are important in order to understand the origin of gravitation and cosmological structures.

Lucio Mayer, Professor for Theoretical Physics at the University of Zurich, and his team are convinced that they have discovered the origin of the first supermassive black holes, which came into being about 13 billion years ago, at the very beginning of the universe. In their article which has appeared in "Nature" magazine, Lucio Mayer and his colleagues describe their computer simulations with which they modelled the formation of galaxies and black holes during the first billion years after the "Big Bang".

According to the current status of knowledge, the universe is approximately 14 billion years old. Recently, research groups discovered that galaxies formed much earlier than assumed until then - namely within the first billion years. The computer simulations from Mayer's team now show that the very first supermassive black holes came into existence when those early galaxies collided with each other and merged.

Galaxies and massive black holes formed very quickly
For more than two decades, science has assumed that galaxies grow hierarchically, i.e. that initially, small masses are pulled together by gravitation, and from them, larger structures form step by step. The researchers at the University of Zurich have now turned that assumption upside down. Mayer says: "Our result shows that large structures such as galaxies and massive black holes formed quickly in the history of the universe. At first glance, this seems to contradict the standard theory with cold dark material which describes the hierarchical building of galaxies." The apparent paradox is explicable according to Lucio Mayer: "Normal matter from which the visible parts of the galaxies and supermassive black holes are formed collapse more strongly than dark material forming quickly the most massive galaxies in the densest regions of the Universe, where gravity begins to form structures earlier than elsewhere. This enables the apparent non-hierarchical formation of galaxies and black holes."

Huge galaxies and supermassive black holes form quickly. Small galaxies - on the other hand, such as our own, the Milky Way and its comparatively small black hole in the centre weighting only 1 million solar masses instead of the 1 billion solar masses of the black holes simulated by Mayer and colleagues - have formed more slowly. As Lucio Mayer explained, the galaxies in their simulation would count among the biggest known today in reality - they were around a hundred times larger than the Milky Way. A galaxy that probably arose from a collision in that way is our neighbouring galaxy M87 in the Virgo cluster, located at 54 million light years from us.

The scientists began their simulation with two large, primary galaxies comprised of stars and characteristic for the beginning of the universe. They then simulated the collision and the merging of galaxies. Thanks to the super-computer "Zbox3" at the University of Zurich and the "Brutus Cluster" from the ETHZ, the researchers were able to observe, at a resolution higher than ever before, what happened next: Initially, dust and condensed gases collected in the centre of the new galaxy and formed a dense disk there. The disk became unstable, so that the gases and the dust contracted again and formed an even more dense region. From that, a supermassive black hole eventually came into existence without forming a star first.

The new findings have consequences for cosmology: The assumption that the characteristics of galaxies and the mass of the black hole are related to each other because they grow in parallel will have to be revised. In Mayer's model, the black hole grows much more quickly than the galaxy. It is therefore possible that the black hole is not regulated by the growth of the galaxy. It is far more possible that the galaxy is regulated by the growth of the black hole. Mayer and his colleagues believe that their research will also be useful for physicists who search for gravitational waves and thus want to supply direct proof of Einstein's theory of relativity. According to Einstein, who received his doctorate in 1906 at the University of Zurich, the merging of supermassive black holes must have caused massive gravitational waves - waves in a space-time continuum whose remains should still be measurable today. The LISA and LISA Pathfinder projects at the ESA and NASA, in which physicists from the University of Zurich are also participants, want to find gravitational waves of that kind. In order to be able to interpret future measurement results correctly, it is important to understand the formation of supermassive black holes in the early time of the universe.

Participants:
Apart from Lucio Mayer and Simone Callegari from the Theoretical Physics department at the University Zurich, further participants in the publication are Stelios Kazantzidis, who received his doctorate at the University of Zurich and is today at the Ohio State University, and Andres Escala, formerly at Stanford University and today at the Universidad de Chile. The research work was financed by the Swiss National Fund SNF, the Center for Cosmology and Astro-Particle Physics at Ohio State and the Kavli Institute for Particle Astrophysics at Stanford University.